This application relies for priority upon Korean Patent Application No. 1999-22497, filed on Jun. 16, 1999, the contents of which are herein incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to an analog-to-digital converter, and more particularly to an analog-to-digital converter operable in a high speed operation.
2. Background of the Invention
An analog-to-digital converter (ADC), converting an analog signal to a digital signal, has many applications in electronic signal processing systems. In particular, the ADC is becoming a more useful component in accordance with an increase of mixed-mode systems. It may be proper for the advanced mixed-mode systems, DVDP (digital video disk player) or DBSR (direct broadcasting for satellite receiver), which would be the foremost applications in the market of household electrical appliances, to be manufactured with low cost and in a one-chip by a CMOS process. For the purpose of that, it is important to provide an ADC capable of conducting a radio frequency (RF) signal of high bandwidth to the system. Such an ADC may be on desire of being operable in high frequency over 100 MHz and having the characteristic of a medium resolution.
Now, among the ADCs of various types proposed by now, xe2x80x9cflashxe2x80x9d ADC has been disclosed in IEEE Journal of Solid State Circuit on December 1979 (Vol. SC-14, pp. 926xcx9c932) by Andrew G. F. Dingwall. In the Dingwall""s flash ADC, an input signal is compared with reference voltages that are of different levels each other and converted into a digital signal at once, by means of amplifiers the number of which corresponds to a predetermined resolution. The flash ADC is advantageous for a fast conversion adaptable to an operating frequency of the one-chip mixed system even though it has a number of circuit elements. As shown in FIG. 1, the flash ADC is composed of reference voltage generator 10, amplifying circuit 20, and latch circuit 30. The reference voltage generator 10 establishes 64 reference voltage levels Vref1xcx9cVref64 from two external reference voltages Vreft and Vrefb between which 65 resistors R1xcx9cR65 are connected in serial. Pre-amplifiers PA1xcx9cPA64 of the amplifying circuit 20 are differential amplifiers receiving the reference voltages Vref1xcx9cVref64 and input voltage Vin. Sixty-four latches L1xcx9cL64 of the latch circuit 30 receives and stores amplified voltages VA1xcx9cVA64 from the pre-amplifiers PA1xcx9cPA64. Each of output data bits DO1xcx9cDO64 generated from the latch circuit 30 is set into six bits by a coding process.
Although the flash ADC of FIG. 1 can accomplish a function of fast and precise data conversion, there are static offsets (most physical) at the pre-amplifiers due to a variation of manufacturing process, external noises, or element parameters. The static offsets would cause the amplifying precise to be worse and a reliable conversion to degrade thereby. In order to eliminate the static offsets, there has proposed of xe2x80x9cauto-zero functionxe2x80x9d in which sampling operations for the difference between the reference voltages Vreft and Vrefb and for the offsets at the pre-amplifiers are employed. However, such auto-zero function does not overcome a limitation of an increase of RC delay time due to sampling capacitors and an operation speed of the amplifier itself, being impossible to accomplish a flash ADC operable in a very high frequency over 100 MHz.
Other techniques to promote the processing speed of the ADC have been proposed in various types. In xe2x80x9cA 200 Msample/S 6 b flash ADC in CMOSxe2x80x9d (ISSCC Dig. Tech. Papers, Febuary 1996, pp. 320xcx9c321), there is provided a specific period assigning to the auto-zero operation and inhibiting a normal operation of the ADC. Nevertheless, that is not available for a general application, just for very specific application.
In xe2x80x9cA CMOS 6-b 200 Msample/s 3V supply converter for a PRML read channel LSIxe2x80x9d (IEEE J. Solid State Circuit, Vol. 31, No. 11, pp. 1248xcx9c1257, September 1996), dummy amplifiers are additionally provided thereto and the static offsets of the amplifiers are removed by activating the dummy and existed amplifiers in alternative operating modes. It needs a complex circuit arrangement in constructing switches for alternatively selecting the amplifiers and circuits for performing timing operations against the switches.
The fast accessing with a high frequency analog signal is determined by bandwidths and slew rates of analog blocks, while the processing speed with a digital signal accords mostly to a latching time, meta-stability, and a propagation delay time. Although the highest frequency of an analog input signal available to be converted into a digital signal is a half of a clock frequency (according to a theoretical analysis), it is substantial to obtain the operating frequency of 10xcx9c20 MHz because of a weak current drivability with the analog circuit block even when the ADC operates in hundreds mega-Hertz. Such insufficient drivability occurs at the top and bottom amplifiers, e.g., PA1 and PA64 in FIG. 1, very seriously. Referring to FIGS. 2Axcx9c2C, the top reference voltage Vref1 as the lowest one is located too low, relative to the input voltage Vin. The small votage difference would cause instability of the amplifying operation due to short periods for high and low levels, PH and PL. Although the current drivability of the amplifiers PA64 and PA1 becomes larger therefrom to increase their amplifying ratios, it is hard to overcome the disharmony between the reference voltages. Since the increase amplifying ratios against the input voltage Vin also makes the swing-depth be larger, the amplifiers PA64 and PA1 can not generate their normal output voltages VA64 and VA1 of high and low levels when the input voltage Vin is higher than their corresponding reference voltages Vref64 and Vref1, as shown in FIGS. 2A and 2B, respectively. The amplifiers PA64 and PA1 can do nothing but generating normal-low and high level output signals, VA64 and VA1 respectively. Furthermore, the larger swing depth due to the increased drivability causes timing discords between the input voltage Vin and the mostly one-sided output signals VA64 and VA1, each of the output signals being incapable of following the oscillation of Vin. The two figures about VA64 and VA1 are extreme cases for the malfunction and timing discord. The nearer the top or bottom reference voltages is easier to cause the malfunctions with the output signals from the amplifiers. All those problems may act as a limitation of carrying out the fast conversion, restricting an allowable frequency that is desirable to be high in the ADC.
Accordingly, it is an object of the invention to provide an ADC capable of performing a faster conversion and a higher resolution.
In order to attain the above object, according to an aspect of the present invention, there is provided an analog-to-digital converter including: a reference generator circuit for generating a plurality of reference voltages; a plurality of amplifying comparators for recieving the reference voltages and analog input signals, each amplifing comparator including: two amplifying paths, each of which has a first amplifier and a second amplifier for alternatively receiving signals from the two paths; a latching comparator having a plurality of latches and receiving output signals from the amplifying comparator; and a digital decoder receiving output signals from the latching comparator and generating a predetermined number of data bits.
Amplifying periods through the two paths are overlapped for predetermined time each other. The amplifying comparator further includes a capacitor for sampling an offset voltage of the first amplifier; a first switch for shorting input and output terminals of the first amplifier; a second switch for resetting the output signals from the first amplifier into a predetermined voltage level in response to a clock signal; and a plurality of switches for controlling a transfer with the reference voltages, the analog input signals, and the output signals from the amplifiers.
The present invention will be better understood from the following detailed description of the exemplary embodiment thereof taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.